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Over the recent decades, an apparent worldwide rise in Harmful Algae Blooms (HABs) has been observed due to the growing exploitation of the coastal environment, the exponential growth of monitoring programs, and growing global maritime transport. HAB species like Alexandrium catenella—responsible for paralytic shellfish poisoning (PSP)—Protoceratium reticulatum, and Lingulaulax polyedra (yessotoxin producers) are a major public concern due to their negative socioeconomic impacts. The significant northward geographical expansion of A. catenella into more oceanic-influenced waters from the fjords where it is usually observed needs to be studied. Currently, their northern boundary reaches the 36°S in the Biobio region where sparse vegetative cells were recently observed in the water column. Here, we describe the environment of the Biobio submarine canyon using sediment and water column variables and propose how toxic resting cyst abundance and excystment are coupled with deep-water turbulence (10−7 Watt/kg) and intense diapycnal eddy diffusivity (10−4 m2 s−1) processes, which could trigger a mono or multi-specific harmful event. The presence of resting cysts may not constitute an imminent risk, with these resting cysts being subject to resuspension processes, but may represent a potent indicator of the adaptation of HAB species to new environments like the anoxic Biobio canyon. [ABSTRACT FROM AUTHOR]

Manned submersible dives in the northwest South China Sea encountered substantial amounts of plastic litter accumulated at the base of scours along the floor of a submarine canyon, which may associate with the depositional behaviors of turbidity currents. In this study, we conduct numerical simulations using field‐scale bathymetry to investigate the relationship between the canyon floor morphology, flow processes, and the locations and sizes of the plastic litter piles. The consistent deposition pattern caused by the numerical turbidity currents with different input parameters indicate that morphology of the canyon may exert a dominant influence on turbidite deposition. This is attributed to a significant reduction in shear velocity as simulated turbidity currents flowing through the scours on the canyon floor. Spatial correspondence between deposits of turbidity currents and plastic litter accumulation suggests that suspended sediments and plastic may undergo simultaneous dynamic processes during the transportation of turbidity currents. Plain Language Summary: The issue of marine plastic litter has attracted wide attention, particularly in terms of its transportation mechanisms and locations of accumulation on the ocean floor. Turbidity currents are subaqueous sediment‐gravity flows that can transport large amounts of sediment, nutrients and pollutants into the deep sea, yet there is sparse research on the dynamics of plastic litter transport under the control of turbidity currents, and its accumulation in the deep sea. Here, we present a series of numerical simulations of turbidity currents in a submarine canyon with various input parameters, when combined with observational data on topography and plastic litter distribution, confirm that turbidity currents constitute a plausible mechanism for the transport of plastic litter and for its accumulation in response to changes in flow associated with scours. In addition, we find that the concavity of the scours is also necessary for plastic litter accumulation, since it induces significant fluctuations in the shear velocity and the corresponding depositional process. Key Points: Numerical simulations were applied to investigate turbidity currents as a cause for plastic litter accumulations in a submarine canyonThe simulated turbidite deposits and observed plastic litter accumulations exhibit a strong spatial correspondenceThe morphology of the canyon floor may exert dominant influence on the plastic litter accumulations in submarine canyons [ABSTRACT FROM AUTHOR]

A collection of swallowers (Chiasmodontidae) was examined in exploration of a contested specific distinction between Chiasmodon niger and C. harteli. The specimens had been taken during four midwater-trawl surveys in The Gully, a submarine canyon and Marine Protected Area (‘MPA’) off the coast of Nova Scotia. Vertebral counts, the principal feature proposed for distinguishing the species, were derived from skeletal images prepared from computed tomography scans. All specimens available for study resembled the description of C. harteli but their morphology was more varied than previously supposed. In addition, the species’ distribution within The Gully was described and the chiasmodontid component of the MPA’s biodiversity established, including the presence of Kali indica – a new record for Canada’s Atlantic waters. [ABSTRACT FROM AUTHOR]

High-resolution 3D seismic data analysis was integrated with a calibrated well and biostratigraphy data to present the first overview of a buried Pleistocene canyon system on the upper slope of the eastern Niger Delta, the Galabor Canyon. Attribute maps of specific horizons allow documenting the changing morphologies and infill lithologies of two main branches of the canyon through two stages of activity separated by a reference horizon dated at 0.99 Ma based on well calibration. At the upper slope, growth faults dissect the canyon heads, the catchment of which encroaches a network of valleys incised on the outer shelf. The canyon fill is composed of muddy channels and mass-transport deposits, largely derived from the collapse of canyon walls and sand-rich bodies forming a tract sourced by shelf-edge deltas at the outlet of the incised valleys. High-density turbiditic processes likely control the distribution of sand bodies along the canyon, ranging from tributary fans on the upper slope to 6 km-wide meander belts on the middle slope. The sandy deposits accumulate in minibasins formed along the canyon path, downstream of the subsiding hanging wall of the growth faults and upstream of shale ridges that damp the flow in the canyon. These results show that canyons can be major targets for sand reservoir exploration on the upper slope of large muddy deltas. [ABSTRACT FROM AUTHOR]

Submarine canyons are hot spots for topography‐internal wave interactions, with elevated mixing contributing to regional water mass transport and productivity. Two velocity time series compare and contrast internal waves deep inside Barkley Canyon to a nearby site on the shelf‐break slope of the Vancouver Island Continental Shelf. Elevation of internal wave energy occurs near topography, up to a factor of 10 above the slope and 100 in the canyon. All frequency bands display strong seasonal variability but weak interannual variability. Diurnal (K1) energy is sub‐inertial, trapped along topography, and forced locally through barotropic motions. Both sites have high near‐inertial (NI) energy linked to wind events, though fewer events are observed deep inside the canyon. At the slope site, near‐inertial energy is attenuated with depth, while in the canyon it is amplified near the bottom. Freely propagating semidiurnal (M2) energy appears focused near critical shelf‐break and canyon floor topography, due to local and remote baroclinic forcing. The high‐frequency internal wave continuum has enhanced near‐bottom energy at both sites (up to 7 × the Garrett‐Munk spectrum), and inferred dissipation rates, ɛ, reaching 10−7 W kg−1 near topography. Dissipation is most strongly correlated with semidiurnal energy variability at both sites, with secondary contributors that are site dependent. Forcing power law fits are ε∼M20.8+ $\varepsilon \sim {M}_{2}^{0.8}+$SubK10.6 ${\text{Sub}}_{{K}_{1}}^{0.6}$ on the slope, and ε∼M21.5+ $\varepsilon \sim {M}_{2}^{1.5}+$ NI0.2 in the canyon. There is also a build‐up of "shoulder" energy (PSh) near the buoyancy frequency, with a power law fit to dissipation of PSh ∼ ɛ0.3 at both sites. Plain Language Summary: Internal waves are sub‐surface waves that can mix ocean water, particularly over rough topography such as that found on continental slopes or in submarine canyons. Mixing is important for understanding ocean circulation, climate, and biological productivity. At Barkley Canyon on Canada's west coast, 4 years of current observations are used to study internal waves at both slope and canyon sites. These long‐term data allow for analysis of seasonal and year‐to‐year trends. The currents follow topography, and near‐bottom internal wave energy is generally increased. The observed seasonal patterns change little year‐to‐year. Internal waves occurring once‐a‐day or less are trapped along the slope, and result from once‐a‐day surface tides. Internal waves associated with the Earth's rotation are driven by wind—with only some events reaching the deep canyon. Internal waves occurring twice‐a‐day may be amplified by topography, and result from twice‐a‐day surface tides and internal waves from other underwater sites. Seasonal trends indicate that larger internal waves transfer energy to smaller, turbulent motions near topography, elevating mixing. There is evidence that this transfer of energy may continue to smaller scales than is typically observed. These findings can support models of mixing near topography and improve understanding of internal wave processes. Key Points: Analysis of 4 years of acoustic Doppler current profiler horizontal velocity data from Barkley Canyon and nearby slopeElevated internal wave energy and mixing near topography, with power law relationships to semidiurnal and site‐dependent secondary forcingBuild‐up of energy near the local buoyancy frequency with a power law relationship to dissipation [ABSTRACT FROM AUTHOR]

Submarine canyons and channels are globally important pathways for sediment, organic carbon, nutrients and pollutants to the deep sea, and they form the largest sediment accumulations on Earth. However, studying these remote submarine systems comprehensively remains a challenge. In this study, we used the only complete-coverage and repeated bathymetric surveys yet for a very large submarine system, which is the Congo Fan off West Africa. Our aim is to understand channel-modifying features such as subaqueous landslides, meander-bend evolution, knickpoints and avulsions by analyzing their morphometric characteristics. We used a new approach to identify these channel-modifying features via morphometric fingerprints, which allows a systematic and efficient search in low-resolution bathymetry data. These observations have led us to identify three morphodynamic reaches within the Congo Canyon-Channel. The upper reach of the system is characterized by landslides that can locally block the channel, storing material for extended periods and re-excavating material through a new incised channel. The middle reach of the system is dominated by the sweep and swing of meander bends, although their importance depends on the channel's age, and the time since the last up-channel avulsion. In the distal and youngest part of the system, an upstream migrating knickpoint is present, which causes multi-stage sediment transport and overspill through an underdeveloped channel with shallow depths. These findings complement previous less- detailed morphometric analyses of the Congo Canyon-Channel, offering a clearer understanding of how submarine canyon-channels can store sediment (due to channel-damming landslides, meander point bars, levee building due to overspill), re-excavate that sediment (via thalweg incision, meander propagation, knickpoint migration) and finally transport it to the deep sea. This improved understanding of the morphodynamics of the Congo Canyon- Channel may help to understand the evolution of other submarine canyon- channels, and assessment of hazards faced by seabed infrastructure such as telecommunication cables. [ABSTRACT FROM AUTHOR]

Turbidity flows can transport massive amounts of sediment across large distances with dramatic, long-lasting impacts on deep-sea benthic communities. The 2016 Mw 7.8 Kaikōura Earthquake triggered a canyon-flushing event in Kaikōura Canyon, New Zealand, which included significant submarine mass wasting, debris, and turbidity flows. This event provided an excellent opportunity to investigate the effects of large-scale natural disturbance on benthic ecosystems. Benthic meiofauna community structure before and after the event was analysed from a time series of sediment cores collected 10 years and 6 years before, and 10 weeks, 10 months, and 4 years after the disturbance. Immediately after the 2016 event abundances of all meiofauna dramatically decreased. Four years later the meiofauna community had recovered and was no longer distinguishable from the pre-event community. However, the nematode component of the community was similar, but not fully comparable to the pre-event community by 4 years after the disturbance. Community recovery was systematically correlated to changes in the physical characteristics of the habitat caused by the disturbance, using physical and biochemical variables derived from sediment cores, namely: sediment texture, organic matter, and pigment content. While these environmental variables explained relatively little of the overall variability in meiofauna community structure, particle size, food availability and quality were significant components. The minimum threshold time for the meiofauna community to fully recover was estimated to be between 3.9 and 4.7 years, although the predicted recovery time for the nematode community was longer, between 4.6 and 5 years. We consider the management implications of this study in comparison to the few studies of large-scale disturbances in the deep sea, in terms of their relevance to the efficacy of the marine reserve that encompasses Kaikōura Canyon, along with potential implications for our understanding of the impacts of anthropogenic seafloor disturbances, such as seabed mining. [ABSTRACT FROM AUTHOR]

The Maps of Geohazard features of Southern Sardinia produced in the framework of the Magic project (MArine Geohazard along Italian Coasts) are here presented. The MaGIC project (Marine Geohazard along the Italian Coasts) had the aim of mapping the geohazard in the Italian seas. The features were derived from the digital elevation model interpretation of the seafloor morphology and shallow sub-surface. From the marine geo-hazards point of view, the main critical elements are represented by gravitational mass processes in the canyon heads, some of which, as in the Toro Canyon, are exposed to seismic triggering. In other cases, we observed that gravitational dynamics connected to fluid leakage processes (pockforms). Large landslides and debris avalanches have been detected in Cagliari Gulf, whereas in eastern upper slope, crescent bedforms, occurring in the eastern sector of the upper slope testify to the upward migration of hyperpycnal erosional structures linked to flows from nearby river inputs.

ABSTRACTWe present the characterization of geohazard-related features of the Ligurian Sea with the map of its physiographic domains (1:250,000 scale) and five maps (1:100,000 scale) of the morphological and morpho-bathymetric elements. These were realized in the framework of the MaGIC (Marine Geohazard along Italian Coasts) project, promoted by the Italian Civil Protection Department. The characterization of submarine geohazards along continental margins, together with mapping of marine geological features are critical for the management of coastal risks. Multibeam bathymetric data combined with high-resolution 2D reflection seismic data, allowed to identify and map the main tectono-sedimentary features related to mass movements along the Ligurian continental margin. A dense grid of seismic profiles allowed to improve the comprehension of the margin structure as well as its recent tectonic activity providing new insights concerning mass wasting processes and active tectonics relationships to plan further, focalized investigations in areas of maximum risk.

High-resolution 3D seismic data analysis was integrated with a calibrated well and biostratigraphy data to present the first overview of a buried Pleistocene canyon system on the upper slope of the eastern Niger Delta, the Galabor Canyon. Attribute maps of specific horizons allow documenting the changing morphologies and infill lithologies of two main branches of the canyon through two stages of activity separated by a reference horizon dated at 0.99 Ma based on well calibration. At the upper slope, growth faults dissect the canyon heads, the catchment of which encroaches a network of valleys incised on the outer shelf. The canyon fill is composed of muddy channels and mass-transport deposits, largely derived from the collapse of canyon walls and sand-rich bodies forming a tract sourced by shelf-edge deltas at the outlet of the incised valleys. High-density turbiditic processes likely control the distribution of sand bodies along the canyon, ranging from tributary fans on the upper slope to 6 km-wide meander belts on the middle slope. The sandy deposits accumulate in minibasins formed along the canyon path, downstream of the subsiding hanging wall of the growth faults and upstream of shale ridges that damp the flow in the canyon. These results show that canyons can be major targets for sand reservoir exploration on the upper slope of large muddy deltas.

Over the recent decades, an apparent worldwide rise in Harmful Algae Blooms (HABs) has been observed due to the growing exploitation of the coastal environment, the exponential growth of monitoring programs, and growing global maritime transport. HAB species like Alexandrium catenella—responsible for paralytic shellfish poisoning (PSP)—Protoceratium reticulatum, and Lingulaulax polyedra (yessotoxin producers) are a major public concern due to their negative socioeconomic impacts. The significant northward geographical expansion of A. catenella into more oceanic-influenced waters from the fjords where it is usually observed needs to be studied. Currently, their northern boundary reaches the 36°S in the Biobio region where sparse vegetative cells were recently observed in the water column. Here, we describe the environment of the Biobio submarine canyon using sediment and water column variables and propose how toxic resting cyst abundance and excystment are coupled with deep-water turbulence (10−7 Watt/kg) and intense diapycnal eddy diffusivity (10−4 m2 s−1) processes, which could trigger a mono or multi-specific harmful event. The presence of resting cysts may not constitute an imminent risk, with these resting cysts being subject to resuspension processes, but may represent a potent indicator of the adaptation of HAB species to new environments like the anoxic Biobio canyon.

We present the characterization of geohazard-related features of the Ligurian Sea with the map of its physiographic domains (1:250,000 scale) and five maps (1:100,000 scale) of the morphological and morpho-bathymetric elements. These were realized in the framework of the MaGIC (Marine Geohazard along Italian Coasts) project, promoted by the Italian Civil Protection Department. The characterization of submarine geohazards along continental margins, together with mapping of marine geological features are critical for the management of coastal risks. Multibeam bathymetric data combined with high-resolution 2D reflection seismic data, allowed to identify and map the main tectono-sedimentary features related to mass movements along the Ligurian continental margin. A dense grid of seismic profiles allowed to improve the comprehension of the margin structure as well as its recent tectonic activity providing new insights concerning mass wasting processes and active tectonics relationships to plan further, focalized investigations in areas of maximum risk. [ABSTRACT FROM AUTHOR]

Submarine canyons and channels are globally important pathways for sediment, organic carbon, nutrients and pollutants to the deep sea, and they form the largest sediment accumulations on Earth. However, studying these remote submarine systems comprehensively remains a challenge. In this study, we used the only complete-coverage and repeated bathymetric surveys yet for a very large submarine system, which is the Congo Fan off West Africa. Our aim is to understand channel-modifying features such as subaqueous landslides, meander-bend evolution, knickpoints and avulsions by analyzing their morphometric characteristics. We used a new approach to identify these channel-modifying features via morphometric fingerprints, which allows a systematic and efficient search in low-resolution bathymetry data. These observations have led us to identify three morphodynamic reaches within the Congo Canyon-Channel. The upper reach of the system is characterized by landslides that can locally block the channel, storing material for extended periods and re-excavating material through a new incised channel. The middle reach of the system is dominated by the sweep and swing of meander bends, although their importance depends on the channel’s age, and the time since the last up-channel avulsion. In the distal and youngest part of the system, an upstream migrating knickpoint is present, which causes multi-stage sediment transport and overspill through an underdeveloped channel with shallow depths. These findings complement previous less-detailed morphometric analyses of the Congo Canyon-Channel, offering a clearer understanding of how submarine canyon-channels can store sediment (due to channel-damming landslides, meander point bars, levee building due to overspill), re-excavate that sediment (via thalweg incision, meander propagation, knickpoint migration) and finally transport it to the deep sea. This improved understanding of the morphodynamics of the Congo Canyon-Channel may help to understand the evolution of other submarine canyon-channels, and assessment of hazards faced by seabed infrastructure such as telecommunication cables.

Male sperm whales are under pressure to grow larger in order to increase their mating opportunities, which could lead them to more efficiently forage in high latitude feeding grounds. Movement patterns of male sperm whales in Nemuro Strait, Japan, were investigated horizontally and vertically using land-based observation and bio-logging methods to determine how they facilitate foraging in the narrow submarine canyon. Eleven tagged whales showed the distinct diel pattern for dive depth, as it was deeper at night than during the day. Five-year data of land-based observation and GPS data from six tagged whales revealed the tendency of whales to change the north-south direction of their horizontal movement every 4–6 h, and this movement direction was not related to the direction of the current. Their periodic heading change is thought to be a consequence of the whales making two round trips each day within the foraging area, one during the day to shallow layers and one during the night to deep layers. These tactics may help the whales to search for prey in this narrow submarine canyon efficiently. Most whales changed their direction of movement in a similar manner, which is probably due to the whales’ tendency to stay close enough to each other to obtain information about the prey environment using the echolocation clicks of other whales. The results emphasize the ability of male sperm whales to adapt their foraging tactics according to the prey environment of their habitat and intense pressure to grow faster may be the drive for this ability. The importance of social cohesion among foraging male sperm whales was also suggested.

Litter is ubiquitous in the ocean, interacting with fauna and causing impacts that are unquantified at present. Mainly sourced from land, marine litter is very persistent, and undergoes slow degradation upon settling on the ocean floor. Submarine canyons contain more litter than other oceanographic features due to hydrological processes, but study of litter in canyons is made difficult by logistical requirements. Monitoring and quantification of marine litter often do not consider interactions between fauna and litter, meaning impacts are largely unconsidered and unknown. Among publications that have reported litter-fauna (L-F) interactions in canyons, the large majority occur in the Mediterranean Sea, and the most reported interaction is of corals entangled in fishing gear. When it occurs, the reporting of L-F interactions is unstandardised, resulting in a lack of global comparison and trend analysis. A standardised, comprehensive framework for the reporting of L-F interactions has been created and includes 6 major categories: entanglement, ingestion, smothering, habitat provision, adaptive behaviour, and encountering (entanglement and smothering occur on abiotic features as well). Use of the framework will aid in research collaboration and creation of a global dataset of L-F interactions. Impacts resulting from interactions are plentiful, most coming from entanglement and smothering.

Enclosed depressions, termed pockmarks, are widespread seafloor morphologies, commonly associated with fluid seepage. This study provides the first detailed documentation of pockmarks offshore the South Westland margin of the South Island/Te Waipounamu, Aotearoa New Zealand. Pockmarks are identified from multibeam bathymetry (25-m grid) through manual and semi-automated selection in water depths of 100–2600 m. Pockmarks are most concentrated at 400–850 m water depth on continental slope areas between submarine canyons. A continuum of pockmark morphologies includes – (1) large (>0.5 km2 area) and irregularly shaped pockmarks above partially infilled channels; (2) small and circular pockmarks (∼100–200 m diameter; ∼0.008–0.03 km2 area) occurring between canyons; and (3) elongated and intermediate size pockmarks, generally oriented along-slope and often occurring above buried sediment waves. Elongated pockmarks appear to have been modified by near-seafloor oceanographic and/or turbidity current flows. Pockmark features occur across many locations around Aotearoa, including both the eastern and western margins. Some similar pockmark morphologies are identified in these different tectonic, sedimentary, and oceanographic settings, suggesting that there may be some similarity in formative mechanisms, but clear mechanisms leading to their formation remain enigmatic. [ABSTRACT FROM AUTHOR]

IntroductionBody size regulates all biological processes, including growth, reproduction, metabolism, trophic interactions, etc., and is the master trait across organisms, populations, and communities. Despite a rich literature on the impacts of human and natural disturbances on body size, a clear knowledge gap is the effect of the submarine canyons on the benthic size structures in the deep sea, hindering our understanding of the ecological processes of these dominant ecosystems on the continental margin.MethodsTherefore, we conducted repeated sediment sampling to compare meiofauna and macrofauna biomass body-size spectrum, growth, metabolism, and size composition from a high-energy submarine canyon, Gaoping Submarine Canyon (GPSC), and the adjacent continental slope off SW Taiwan. The GPSC is a dynamic ecosystem connected to a high sediment-yield small mountain river subjected to strong internal-tide energy, swift bottom currents, frequent mass wasting events, and high terrestrial sediment inputs.ResultsWe found that the meiofauna and macrofauna were characterized by relatively larger individuals dominating on the slope to smaller ones dominating in the canyon. As a result, the community biomass, secondary production, and respiration were depressed with distinctive biomass-size composition in the canyon compared to the non-canyon slope. The environmental factors related to internal tide disturbance (i.e., bottom current velocity, duration of sediment erosion, or low light transmission) substantially influence the body size composition of the canyon benthos, while food supplies (i.e., TOC and C/N ratio) and sediment characters (i.e., grain size and porosity) correlated closely with the slope communities.DiscussionWe concluded that the disturbed condition in the GPSC may have wiped out or depressed the local benthic assemblages, and only the smaller, more resilient species could persist. Our results also highlight that the alterations of the canyon benthic community could be a reference to deep-sea ecosystems under anthropogenic disturbances or global climate change.

Vertical walls of submarine canyons represent features of high conservation value that can provide natural areas of protection for vulnerable marine ecosystems under increasing anthropogenic pressure from deep-sea trawling. Wall assemblages are spatially heterogeneous, attributed to the high environmental heterogeneity over short spatial scales that is a typical feature of canyons. Effective management and conservation of these assemblages requires a deeper understanding of the processes that affect faunal distribution patterns. Canyons are recognised as sites of intensified hydrodynamic regimes, with focused internal tides enhancing near-bed currents, turbulent mixing and nepheloid layer production, which influence faunal distribution patterns. Faunal patterns also respond to broad-scale hydrodynamics and gradients in water mass properties (e.g. temperature, salinity, dissolved oxygen concentration). Oscillating internal tidal currents can advect such gradients, both vertically and horizontally along a canyon's walls. Here we take an interdisciplinary approach using biological, hydrodynamic and bathymetry-derived datasets to undertake a high-resolution analysis of a subset of wall assemblages within Whittard Canyon, North-East Atlantic. We investigate if, and to what extent, patterns in diversity and epibenthic assemblages on deep-sea canyon walls can be explained by spatial and temporal variability induced by internal tides. Vertical displacement of water mass properties by the internal tide was calculated from autonomous ocean glider and shipboard CTD observations. Spatial patterns in faunal assemblage structure were determined by cluster analysis and non-metric Multi-Dimensional Scaling plots. Canonical Redundancy Analysis and Generalised Linear Models were then used to explore relationships between faunal diversity and assemblage structure and a variety of environmental variables. Our results support the hypothesis that internal tides influence spatial heterogeneity in wall faunal diversity and assemblages by generating both spatial and temporal gradients in hydrodynamic properties and consequently likely food supply.

This paper carried out laboratory experiments to study evolution of internal solitary waves (ISWs) over submarine canyons with a combination of PIV (particle image velocimetry) and PLIF (planar laser-induced fluorescence) techniques. Taking canyon angle θ and collapse height ∆H as variables, Froude number Fr, head position, energy loss, vorticity field and turbulence intensity when ISWs propagate to the canyon were analyzed. According to the Froude number Fr values, the study cases can be divided into three types: Fr > 1.7 means complete internal hydraulic jump (IHJ); 1 < Fr < 1.7 denotes wavy IHJ and Fr < 1 represents no IHJ. The greater canyon angle, collapse depth and amplitude of the incident wave more easily generate IHJs, which can lead to more energy loss, greater vorticity and turbulence intensity in the canyon area. Among all canyon cases, vorticity and turbulence intensity of the no IHJ case showing an obvious bimodal distribution are smaller than IHJ cases. For wavy IHJ, the energy dissipation is not obvious, and the average turbulent intensity performs a "sharp unimodal distribution". Complete IHJ cases last for a long time and cause violent mixing, the average turbulent intensity is the largest and its distribution presents a "gentle single peak" pattern. For the 180° conditions (no canyon cases), less energy is delivered to the reflected wave and more energy is dissipated near the terrain, so the energy loss is the largest in comparison to other conditions. These findings will deepen our understanding of the evolution mechanisms of ISWs propagating over submarine canyons. [ABSTRACT FROM AUTHOR]

Previous gas hydrate production tests conducted by the Guangzhou Marine Geological Survey (GSGM) in 2017 and 2020 indicated the great potential of gas hydrates in the Shenhu Sea area in the Pearl River Mouth Basin (PRMB), China. In this study, the effects of deposition processes in submarine canyons and the distribution of gas chimneys on gas hydrate accumulation were investigated using high-resolution two- dimensional (2D) and three-dimensional (3D) seismic data. Four intact submarine canyons were identified in the study area. Five deepwater depositional elements are closely related to submarine canyons: lateral accretion packages (LAPs), basal lags, slides, mass transport deposits (MTDs), and turbidity lobes. MTDs and lobes with multiple stages outside the distal canyon mouth reveal that the sedimentary evolution of the canyon was accompanied by frequent sediment gravity flows. Gas chimneys originating from Eocene strata are generally up to 3 km wide and distributed in a lumpy or banded pattern. The analysis of seismic attributes confirmed fluid activity in these gas chimneys. Gas hydrates are mainly distributed in ridges among different canyons. Based on the gas sources of gas hydrates and depositional evolution of submarine canyons, depositional processes of sediment gravity flows in submarine canyons and the distribution of gas chimneys significantly affect the accumulation of gas hydrates. Based on these findings, this study establishes a conceptional model for the accumulation of gas hydrate, which can provide guidance in the prediction for favorable gas hydrates zones in the area and nearby. [ABSTRACT FROM AUTHOR]

The largest canyons on Earth occur on the seafloor, and seabed sediment flows called turbidity currents play a key role in carving these submarine canyons. However, the processes by which turbidity currents erode submarine canyons are very poorly documented and understood. Here we analyse the first detailed time-lapse bathymetric surveys of a large submarine canyon, and its continuation as a less-deeply incised channel. These are also the most comprehensive time-lapse surveys before and after a major canyon-channel flushing turbidity current. These unique field data come from the Congo Submarine Fan offshore West Africa, where canyon flushing turbidity currents between 2019 and 2020 eroded ~2.65 km3 of seabed sediment, as they travelled for over 1100 km at speeds of 5–8 m/s. This eroded sediment volume is equivalent to ~19–33 % of global sediment flux from all rivers to the oceans. The time-lapse surveys cover 40 % of the 1100 km long submarine canyon-channel. They show that erosion was predominantly (94 %) along the canyon-channel axis, with only 6 % from failures along canyon or channel flanks. However, erosion along the canyon-channel floor was very patchy; some areas were eroded to depths of 10–20 m, whilst intervening areas showed no significant change. Knickpoints with up-slope migrating headscarps account for 22 % of the total eroded volume. One knickpoint in the deep-sea channel migrated by 21 km in one year, making it the fastest moving submarine knickpoint yet documented. Most (62 %) eroded sediment was in zones extending across the canyon or channel floor, without distinct headscarps as is the case for knickpoints. Erosion restricted to outer bends only comprised 10 % of the total, suggesting processes of erosion differ significantly from meandering rivers in which outer bend erosion is more important. Patchy seabed erosion appears to be mainly due to flow-bed processes (e.g. knickpoints), but spatial variations in seabed sediment properties may also play a role. The irregular seabed erosion occurs despite near-uniform flow speeds observed between moorings and submarine cable breaks with spacing of tens to hundreds of kilometers. Patchy and localised erosion has important implications for assessing hazards to seabed telecommunication cables, which are more likely to break in areas of deep erosion, and for creating appropriate numerical models of seabed erosion and turbidity current behaviour, or how to interpretate ancient submarine canyons and channels in rock outcrops. • First detailed time-lapse surveys of a major submarine canyon-channel system. • Surveys capture patterns of erosion due to powerful canyon-channel flushing flows • Seabed erosion is highly localised and patchy, despite relatively uniform flow speed • Knickpoints can migrate at up to 21 km per year, but not all knickpoints migrated. • Localised erosion may explain why some seabed cables break, but others survive. [ABSTRACT FROM AUTHOR]

IntroductionSubmarine canyons are incisive morphologies that play an important role in the exchange between shallow and deep waters. They interact with the general circulation and induce a specific circulation locally oriented by the morphology. The characteristics of the physical processes at play, the way they interact with each other and the influence of extreme events is still an open question as few observations are available. To answer this question and to improve the representation of submarine canyons in numerical models, it is key to understand the specific circulation patterns and their transitions in these specific environments.MethodsThis paper presents observations of currents, temperature and turbidity along the Cassidaigne canyon, northwestern Mediterranean Sea. Two oceanographic cruises carried out in 2017 and 2019 gathered data from the outer shelf and canyon head at 100-400 m depth to the base of the continental slope at 1900 m depth.Results and DiscussionThe circulation in the Cassidaigne area is subject to upwelling and downwelling-favorable winds, to the Northern Current and its associated mesoscale structures and is oriented by the local morphology. Upwellings occur both during stratified and non-stratified conditions. They are triggered by a wind forcing higher than 14 m s–1 and their consecutive relaxations are marked by a counter-current. Near the canyon head and on the shelf, the current orientation depends on the stratification, the wind, the bottom morphology and the general circulation. The mesoscale variability of the Northern Current can lead to its intrusion over the shelf leading to barotropic cross currents over the canyon. At 1700 m depth, a quasi-permanent residual up-canyon flow is observed in a narrow gorge area and can be extrapolated to the canyon body. Finally, turbidity currents were observed for the first time in connection with upwelling events, suggesting the key role of canyons’ internal hydrodynamics on shelf sedimentary processes.

Marine microplastics are widely distributed in deep-sea sedimentary environments and are altering sediment compositions and ecological conditions on the seafloor. However, the relation between the distribution of microplastics in deep-sea sediments and the sedimentary dynamic conditions is poorly understood. In this study, we collected surface sediments from some typical geomorphological units (sand dune, sediment drift, and submarine canyon channel/levee) in the northern South China Sea to study composition and distribution of the deep-sea microplastics and their controlling factors. The results show that the microplastic abundance in surface sediments ranges from 19 to 347 p·kg–1, and the identified microplastics consist of 10 types, including dominant polycarbonate (29%), polyethylene (27%), polyester fiber (16%), polyvinyl chloride (13%), and polypropylene (7%), and minor polyethylene terephthalate resin, acrylonitrile-butadiene-styrene, epoxy resin, hydrocarbon resin, and acrylic. The source analysis shows that the deep-sea microplastics may be influenced by riverine inputs from Taiwan and South China. In addition, the microplastic spatial distribution shows that the sand dune and canyon channel contain the highest abundances (136–347 p·kg–1) and more types (4–6 types) of microplastics, which are dominated by relatively high-density polycarbonate or polyvinyl chloride. The canyon levee contains the lowest abundances (19–132 p·kg–1) and less types (1–3 types) of microplastics, which are dominated by relatively low-density polyester fiber or polyethylene. Nevertheless, the microplastic composition of the sediment drift is between those of the canyon channel and the canyon levee. The abundance and polymer type (density) of microplastics all increase with the increased mean grain size of detrital sediments, which represents the progressively enhanced intensity of sedimentary dynamic conditions. We therefore infer that the sedimentary dynamic conditions control the composition and distribution of microplastics in the deep-sea sediments. This study highlights that some deep-sea environments with stronger sedimentary dynamic conditions may accumulate more microplastics, which is of great significance for evaluating the storage and ecological damage of deep-sea microplastics.

This note analyzes for the first time the geomorphology and sedimentary features of the Simpson Submarine Canyon (SSC), located between Chiloé Island and the Taitao Peninsula. For that purpose, multibeam bathymetric data were obtained in 2018. The SSC has a unique orientation compared to most canyons on the Chilean margin. Slope escarpments, topographic irregularities, and sinuosity of the canyon could be associated with regional tectonics. Sediment transport and deposition along the axis define the transversal morphology. On the canyon walls, erosion and transport processes dominate, evidenced by gullies, channels, and mass removals, which leave debris on the axis. We report a large landslide from a canyon wall, which could be due to a highenergy event such as an earthquake; and the generation of a large sediment wave field outside the canyon mouth, indicating a great activity by sedimentary processes. All the above could indicate that the canyon is continuously evolving. [ABSTRACT FROM AUTHOR]

The submarine canyons are an important clue to study the evolution process of seafloor geomorphology and they generally indicate the significant linear grooves on the seafloor related to seafloor geodynamic erosion during the evolution of geomorphology. The submarine canyons or canyon groups are not only the channels for the sediment transport from shallow sediments with land-based sources to the deep sea in the sediment source-sink system, but also a key temporary sediment deposition area to study sediment transport patterns and the evolution of submarine geomorphology. In this paper, we processed and analyzed the multibeam bathymetry data acquired in the South China Sea continental margin by the research vessel "Dongfanghong 3" in 2020. Based on fine submarine geomorphological features identified from multibeam bathymetry data, we construct the formation pattern of the cyclic steps. The six cyclic steps (wavelengths of 1–6 km and wave heights of 19–81 m) are found in the lower section of the Penghu canyon and they appeared at the conjunction part of the Penghu and the Taiwan canyon. Based on location and the wavelength variations of the cyclic steps, we propose that the cyclic steps are formed by turbidity current flow along the Penghu and the Taiwan canyons. The axis of the cyclic step CS4–CS6 is shifted westward by about 5° compared to the axis of the cyclic step CS1–CS3. The inconsistency in the axis direction of the cyclic steps CS1–CS3 and CS4–CS6 suggests that is where the migration of the Penghu canyon occurred. [ABSTRACT FROM AUTHOR]

Submarine canyons are important conduits of sediment and organic matter to deep-sea environments, mainly during high-energy natural events such as storms, river floods, or dense shelf water cascading, but also due to human activities such as bottom trawling. The contributions of natural and trawling-induced sediment and organic matter inputs into Palamós Canyon (NW Mediterranean) were assessed from three instrumented moorings deployed in the axis and northern flank of the canyon covering the trawling closure (February) and the trawling season (March-December) of 2017. During the trawling closure, large sediment fluxes with high contents of labile marine organic matter content were registered in the canyon axis, associated to storm resuspension on the shelf that coincided with dense shelf water cascading and high surface water productivity. Although no major natural sediment transport events occurred during the following spring and summer months, near-daily trawling-induced sediment gravity flows were recorded in the northern flank mooring, placed directly below a fishing ground, which sometimes reached the canyon axis. Compositionally, the organic matter transferred by trawling resuspension was impoverished in the most labile biomarkers (fatty acids, amino acids, and dicarboxylic acids) and had a high degree of degradation, which was similar to surficial sediment from the adjacent fishing ground. Trawling resuspended particles masked the transfer of organic matter enriched in labile biomarkers that naturally occur during the quiescent summer months. Overall, bottom trawling enhances the magnitude of particle fluxes while modifying its organic carbon composition, increasing the re-exposure and transfer of degraded organic carbon and potentially affecting benthic communities that rely on the arrival of fresh organic matter.

Coastal submarine canyons are expected to be sites of enhanced turbulent mixing which create productive fishing areas in coastal waters. Recent studies suggested the potential role of diurnal coastal trapped waves (CTWs) in inducing significant turbulent mixing in coastal submarine canyons poleward of 30° where the diurnal tide becomes subinertial. Nevertheless, the detailed physical processes responsible for the generation and dissipation of CTWs in such submarine canyons have hardly been investigated so far. In the present study, to investigate the turbulent mixing processes associated with diurnal CTWs in submarine canyons, we carry out high‐resolution (Δx, Δy = 1/240°) non‐hydrostatic three‐dimensional numerical experiments focusing on Suruga Bay located at the north end of Suruga Trough, Japan. The numerical experiments show that diurnal (subinertial) tidal currents over the Izu Ridge generate CTWs which propagate cyclonically along the coast of Suruga Bay. In the middle of Suruga Bay, bottom‐intensified flow due to the diurnal CTWs interacts with rough seafloor topography to excite upward propagating internal lee waves, which create mixing hotspots extending high above the seafloor. The resulting diurnal (subinertial) tidal energy dissipation in Suruga Bay is found to be much larger than that associated with semidiurnal (superinertial) internal tides. Furthermore, it is found that baroclinic energy flux based on a simple barotropic‐baroclinic decomposition severely underestimates the wave energy flux in areas with high CTW activity. These results strongly indicate that the existence of CTWs might be a key factor to clarify the dissipation/mixing processes in submarine canyons. Plain Language Summary: Recent microstructure measurements indicate that strong turbulent mixing in submarine canyons causes the increase of biological productivity through upwelling of deep water. In the present study, we numerically investigate the mixing processes in submarine canyons. It is found that, in Suruga Bay located at the north end of Suruga Trough, Japan, coastal trapped waves (CTWs) propagating cyclonically along the coast of Suruga Bay interact with rough seafloor topography to excite small‐scale internal waves that break inducing significant mixing. These results suggest that numerical models that resolve small‐scale rough seafloor topography are indispensable to numerically investigate the turbulent mixing processes in submarine canyons. Key Points: The mixing processes associated with diurnal coastal trapped waves (CTWs) in Suruga Bay are investigated through numerical experimentsDiurnal CTWs propagating into Suruga Bay interact with small‐scale topography to excite internal lee waves that produce mixing hotspotsAnalyses based on a barotropic‐baroclinic decomposition may cause a large error in estimating energy flux in areas with high CTW activity [ABSTRACT FROM AUTHOR]

In this paper we present geomorphological hazard mapping of the San Vito Peninsula offshore (Sicilian margin of Tyrrhenian Sea), characterised by a very narrow continental shelf and a very dipping, tectonically active continental slope, no far from a seismogenic belt. The data set consists of morpho-bathymetric models achieved by means of multibeam survey and scattered high resolution seismic profiles. The morpho-bathymetric study highlighted some potential sites of geomorphological hazards, the most representative of which are: (i) two canyons developing from the upper slope off San Vito Cape down to the Erice basin; (ii) a dense network of small incisions across the upper slope; (iii) a very active landslide, developed onland and extending in the continental shelf close to the Scopello village. The geomorphological features that can generate submarine hazard are represented by: unstable sediment packages into canyon heads or along the upper slope; submarine canyons with regressive erosion; coastal landslides.

Three main types of factors commonly control the nature of the clasts, the arrangement of the distinctive lithologies, and the general architecture of turbidite systems: sedimentation rate and carbonate production; climates and glacio-eustatism; and morphology and tectonics. The coexistence of adjacent systems of distinctive nature is, however, scarcely documented, and the relative influence of each factor needs better constrain. In the Northern Lesser Antilles Segment (NLAS), carbonate and siliciclastic sediment sources coexist within a 150 km lateral distance, with carbonate platforms lying onto a volcanic substratum, and by a succession of spurs and triangular valleys that are bounded by active normal faulting. To better understand the factors controlling sedimentary processes from the carbonate platform sources to the deep-sea sinks, we used backscatter, bathymetry, multichannel seismic, and sub-bottom profiles. Sedimentary systems are dominated by siliciclastic input (by retrogressive erosion of confined canyons affecting the volcanic slope), carbonate input (by carbonate sediment transported by oceanic- and wind-driven submarine currents beyond the leeward edges of carbonates platforms), or both. In the mixed systems, the retrogressive erosion of the canyon head determines the nature of the source (volcanic on the slope, carbonate when the canyons reach the shelf edge). Glacio-eustatism has a key role in carbonate availability on the platform, as attested by the presence of drowned platforms. The main contribution of this study is the identification of the major role that tectonic activity plays in the short-distance coexistence of the distinctive sedimentary systems since fault-bounded V-shaped valleys in map view offer alternating leeward and windward edges favoring carbonate or mixed systems. Additionally, the steep slope gradient induced by normal faults and regional subsidence seems to be the main factor controlling sediment dispersal. It causes multiple line sources and the dispersion of gravity-driven currents under the effect of hydraulic jumps, thus preventing the formation of a channelized system. Our study provides a modern analog of adjacent systems dominated by distinctive lithologies in a tectonically active area. The results appear particularly appropriate to decipher the nature of ancient source-to-sink systems dominated by complex tectonics, paleo-bathymetry, and sediment routings.

Submarine canyons provide a conduit for shelf-slope exchange via topographically induced processes such as upwelling and downwelling. These processes in the Wilmington Canyon, located along the shelf-break of the Mid-Atlantic Bight (MAB), have not been previously studied, and the associated hydrographic variability inside the canyon and on the adjacent shelf are largely unknown. Observations from an underwater glider deployed in Wilmington Canyon (February 27 - March 8, 2016), along with wind and satellite altimetry data, showed evidence for a wind-driven canyon upwelling event followed by a subsequent downwelling event. Next, a numerical model of the MAB was developed to more fully represent these two events. Modeled results showed that under upwelling-favorable winds during February 25 - March 3, sea level increased seaward, shelf currents flowed northeastward, and canyon upwelling developed. Then under downwelling-favorable winds during March 4-7, sea level increased landward, shelf currents flowed southwestward, and canyon downwelling developed. Modeling experiments showed that canyon upwelling and downwelling were sub-tidal processes driven by winds and pressure gradients (associated with SSH gradients), and they would occur with or without tidal forcing. During the upwelling period, slope water originating from 150-215 m depths within the canyon (75 m below the canyon rim), was advected onto the shelf, forming a cold and dense canyon-upwelled slope-originated overflow water at the bottom of the outer shelf (75-150 m isobaths). The dense overflow current flowed was directed northeastward and expanded in the cross-shelf direction. It was 5-20 km wide and 10-30 m thick. The estimated volume of the plume overflow water exceeded 6×109 m3 at peak. The density front at the shoreward side of the dense overflow water caused a subsurface baroclinic frontal jet, which flowed northeastward and along-shelf with maximum speed exceeding 0.5 m/s. In the ensuing downwelling event, a portion of the previously upwelled dense water was advected back to the canyon, and then flowed down-slope in the upper canyon in ~0.3 m/s bottom-intensified currents. Dynamical investigation of the overflow current showed that its evolution was governed by unbalanced horizontal pressure gradient force in the cross-shelf direction and that the current was geostrophic.

Our recent echo-character analysis of the Tierra del Fuego Continental Margin, focusing on Sloggett Canyon and the interfluve with Valentín Canyon, provides insights into contemporary sedimentary processes. Utilizing high-resolution seismic profiles obtained from the R/V Austral in 2017, we categorized sub-bottom echoes into 10 distinct types, shedding light on various processes such as hemipelagic deposition, mass wasting events, glacial influence, and the impact of bottom water masses. By integrating data from the GLORYS12V1 model (1993–2020), CTDs from the World Ocean Database, and sediment samples, we analyze the interaction between bottom currents, seafloor topography, and sediment characteristics. We conclude that the flow of the Subantarctic Water Mass acts as an active transport of coarser sediment from the continental shelf into the canyon, maintaining the Sloggett Canyon's activity, while the flows of the Upper and Lower Circumpolar Water Masses contribute to contourite formation along the eastern canyon flank and the erosion of the lower interfluve, leading to the generation of distinctive longitudinal scours. Additionally, in the continental rise, the interaction of water masses with the seafloor influences the redistribution of the deep-sea fans from the Sloggett and Valentín canyons towards the northeast. This study significantly enhances our understanding of the sedimentary dynamics in this area, establishing the basis of the sedimentary distribution for future interdisciplinary studies and for setting a new baseline in marine protected areas. • We identified 10 echo-characters in the area encompassing Sloggett Canyon and its interfluve with Valentín Canyon. • Longitudinal scours on the lower interfluve result from bottom current erosion and deposition. • The eastern flank of the Sloggett Canyon is under the influence of hemipelagic sedimentation and mass wasting events. • Sediment is transported by flows of the Subantarctic Water Masses from the continental shelf into the canyon head. [ABSTRACT FROM AUTHOR]

Submarine canyons are primary conduits for turbidity currents transporting terrestrial sediments, nutrients, pollutants and organic carbon to the deep sea. The concavity in the longitudinal profile of these canyons (i.e. the downstream flattening rate along the profiles) influences the transport processes and results in variations in turbidite thickness, impacting the transfer and burial of particles. To better understand the controlling mechanisms of canyon concavity on the distribution of turbidite deposits, here we investigate the variation in sediment accumulation as a function of canyon concavity of 20 different modern submarine canyons, distributed on global continental margins. In order to effectively assess the isolated impact of the concavity of 20 different canyons, a series of two-dimensional, depth-resolved numerical simulations are conducted. Simulation results show that the highly concave profile (e.g. Surveyor and Horizon) tends to concentrate the turbidite deposits mainly at the slope break, while nearly straight profiles (e.g. Amazon and Congo) result in deposition focused at the canyon head. Moderately concave profiles with a smoother canyon floor (e.g. Norfolk-Washington and Mukluk) effectively facilitate the downstream transport of suspended sediments in turbidity currents. Furthermore, smooth and steep upper reaches of canyons commonly contribute to sediment bypass (i.e. Mukluk and Chirikof), while low slope angles lead to deposition at upper reaches (i.e. Bounty and Valencia). At lower reaches, the distribution of turbidite deposits is consistent with the occurrence of hydraulic jumps. Under the influence of different canyon concavities, three types of deposition patterns are inferred in this study, and verified by comparison with observed turbidite deposits on the modern or paleo-canyon floor. This study demonstrates a potential difference in sediment transport efficiency of submarine canyons with different concavities, which has potential consequences for sediment and organic carbon transport through submarine canyons. • The impact of submarine canyon concavity on turbidite deposition was assessed. • Distribution of turbidite deposits varies with changes in canyon concavity. • Three distinct deposition patterns were identified. • The recognized deposition patterns align well with the observed turbidite deposits. [ABSTRACT FROM AUTHOR]

Knowledge of the spatial distribution and habitat preferences of marine top‐predators is essential for monitoring their populations. The summertime abundance of male sperm whales (parāoa, Physeter macrocephalus) at the Kaikōura Canyon (New Zealand) has almost halved over the last three decades, possibly reflecting a shift in distribution away from this foraging ground. To better understand the seasonal drivers of habitat use by sperm whales, we recorded presence and absence locations of foraging whales during acoustic‐visual surveys, in conjunction with water‐column oceanographic sampling. Season‐specific generalized additive models (GAM) indicated that whale distribution was best predicted by seafloor depth, thermal stratification in the water‐column, and slope gradient and orientation. Habitat use differed between summer and winter, consistent with patterns in diving behavior and stable isotope ratios, suggesting seasonal fluctuations in prey targeted by sperm whales. Our results advance understanding of fine‐scale habitat use by sperm whales, with new insights into the ecological drivers of seasonal fluctuations in foraging distribution. Our study demonstrates the importance of quantifying water‐column processes in the study of habitat preferences by deep‐diving predators. We also identify thermal stratification as an oceanographic feature susceptible to climate change, which could contribute to the decline in sperm whale abundance at Kaikōura. [ABSTRACT FROM AUTHOR]

Coastal dynamics are the result of several processes controlling the balance between sediment input and output over time. The beach system is not always able to maintain a neutral coastal balance due to natural and anthropogenic causes. We present an integrated marine geology, geomorphological and sea-level rise analysis in the coastal sector between Torre delle Ciavole and Capo Calavà (North-Eastern Sicily, Italy).This sector is characterized by high uplift rates and frequent seismicity (mainly generated by the very active Vulcano-Tindari Fault System), promoting the development of mass-wasting processes in the coastal and offshore sectors. A main erosive feature observed in the area is the head of the Gioiosa Marea submarine canyon, located at some meters of depth, few hundred meters far the coastline. The main morphological features of the canyon were reconstructed through the analysis of high-resolution multibeam data, indicating that the canyon is active, as also testified by the comparison of time-lapse aerial photos. Due to this active setting, the study area is exposed to multiple geohazards, among which we deal with: (1) retrogressive instability at the head of the Gioiosa Marea submarine canyon, (2) coastal erosion favored by the downlope funnelling of littoral drift at the canyon head, (3) flooding scenario at 2100 using the IPCC (Intergovernmental Panel on Climate Change) and Rahmstorf sea-level projections. The consequences associated with these geohazards are amplified by the strong anthropization pressures occurring along in this sector. Our results provide key insights regarding the future scenarios of this coastal sector, revealing the effects of the retrogressive activity associated with the canyon head on the coastal strip. We also present the first management tool for the application of forecasting studies by local administrations. [ABSTRACT FROM AUTHOR]

Submarine canyons are efficient sediment transport pathways from shallow marine areas to deep sea. Along active margins, large tsunamis are a trigger for sediment transport to deep sea. However, sediment transport through submarine canyons by such extreme wave events in the carbonate depositional environments has not been fully understood. Large tsunamis have repeatedly struck the coral reef islands of the southern Ryukyu Islands and have transported large boulders composed of coral from the reef to shore. In this study, we examined sediment cores collected near the mouths of submarine canyons and basin floor on the southern Ryukyu arc’s forearc. The presence of coarse calciturbidites containing coral, molluskan fragments, and coral reef benthic foraminifera was limited on the submarine fan formed at the mouth of the reef-connected and shelf-incised submarine canyon. In cores collected near the mouth of shelf-incised submarine canyons with no reef connection and slope-confined canyons, no coarse calciturbidite is observed. Few calciturbidites were found in more downslope cores, implying that most calcareous grains derived from shallow marine areas were deposited on this fan. Depositional intervals of turbidites were calculated to be several hundred–a few thousand years, which agree with the recurrence intervals of large tsunamis estimated from onshore tsunami deposits, based on radiocarbon dates in hemipelagic mud intervals. No temporal change in the depositional intervals of calciturbidites in the cores from the submarine fan since the last glacial maximum. Therefore, the tsunamis may be an important mechanism for surface sediment reworking in coral reefs, and shelf-incised and reef-connected submarine canyons can play an important role in the efficient transport of shallow marine calcareous grains to the deep sea.

Gulf Stream warm‐core rings (WCRs) impinging onto the Mid‐Atlantic Bight (MAB) shelf edge can induce substantial water exchange between the shelf and slope seas. Combining satellite imagery and idealized ocean models, this study investigates the long‐neglected influence of submarine canyons on the WCR impingement process. Satellite images show onshore intrusion of the WCR water concentrated near the MAB shelf‐break canyons, indicating canyon‐induced enhancement of cross‐shelf exchange. Model simulations of the ring‐canyon interaction qualitatively reproduce the observed pattern and show greatly enhanced vertical motions and cross‐shelf transport in a canyon. The ring‐induced transient flow in a canyon resolved by the model is consistent with the three‐dimensional canyon circulation driven by ambient along‐slope steady flows as depicted in the literature. Cross‐isobath flows occur over both canyon slopes with a strong upwelling onshore flow over the slope upstream to the coastal‐trapped wave propagation (the upwave slope) and a weak downwelling offshore flow over the downwave slope. To conserve potential vorticity, a subsurface‐intensified cyclonic eddy is formed inside the canyon, which interacts with the sloping bottom and enhances the upwelling onshore flow over the upwave slope. The upwelled deep ring water is transported either back offshore by the ring‐edge current on the upwave side of the canyon or across the canyon onto the downwave shelf forming a localized bulge pattern. While the former is an ephemeral onshore transport process, the latter represents a more sustained onshore transport of the ring water, both of which have major implication for ecosystem dynamics at the shelf edge. Plain Language Summary: Waters on the Mid‐Atlantic Bight continental shelf and in the neighboring deep ocean have dramatically different physical and biological properties. Warm vortices with diameters of 100–150 km, so‐called warm‐core rings, broken off the unstable Gulf Stream can migrate in the deep ocean and interact with the shallower shelf edge resulting in water exchange between the shelf and the deep ocean. This study investigates the influence of shelf‐edge submarine canyons on the ring‐induced cross‐shelf water exchange. Satellite‐measured sea surface temperature indicates that shelf‐edge canyons enhance onshore intrusion of the offshore ring water onto the continental shelf. Computer models with simplified canyon topography and warm‐core rings qualitatively reproduce the satellite‐observed ring‐canyon interaction, and confirm that canyons can enhance cross‐shelf water exchange during the impingement of a warm‐core ring onto the shelf edge. The models also reveal a complex three‐dimensional circulation pattern inside the canyon, including a confined deep eddy and a strong upward flow on one of the canyon side slopes. The latter can transport a substantial amount of nutrient‐rich deep offshore water into the surface layer on both sides of the shelf edge and thus stimulate biological productivity on the continental shelf and also in the deep ocean. Key Points: Satellite imagery shows localized onshore intrusion of Gulf Stream warm‐core ring water at the Mid‐Atlantic Bight shelf‐break canyonsIdealized model simulations reveal the formation of a subsurface eddy inside the canyon when a warm‐core ring impinges onto the canyonInteraction of the subsurface eddy with the canyon topography enhances intrusion of the warm‐core ring water onto the continental shelf [ABSTRACT FROM AUTHOR]

This paper carried out laboratory experiments to study evolution of internal solitary waves (ISWs) over submarine canyons with a combination of PIV (particle image velocimetry) and PLIF (planar laser-induced fluorescence) techniques. Taking canyon angle θ and collapse height ∆H as variables, Froude number Fr, head position, energy loss, vorticity field and turbulence intensity when ISWs propagate to the canyon were analyzed. According to the Froude number Fr values, the study cases can be divided into three types: Fr > 1.7 means complete internal hydraulic jump (IHJ); 1 < Fr < 1.7 denotes wavy IHJ and Fr < 1 represents no IHJ. The greater canyon angle, collapse depth and amplitude of the incident wave more easily generate IHJs, which can lead to more energy loss, greater vorticity and turbulence intensity in the canyon area. Among all canyon cases, vorticity and turbulence intensity of the no IHJ case showing an obvious bimodal distribution are smaller than IHJ cases. For wavy IHJ, the energy dissipation is not obvious, and the average turbulent intensity performs a “sharp unimodal distribution”. Complete IHJ cases last for a long time and cause violent mixing, the average turbulent intensity is the largest and its distribution presents a “gentle single peak” pattern. For the 180° conditions (no canyon cases), less energy is delivered to the reflected wave and more energy is dissipated near the terrain, so the energy loss is the largest in comparison to other conditions. These findings will deepen our understanding of the evolution mechanisms of ISWs propagating over submarine canyons.

Previous gas hydrate production tests conducted by the Guangzhou Marine Geological Survey (GSGM) in 2017 and 2020 indicated the great potential of gas hydrates in the Shenhu Sea area in the Pearl River Mouth Basin (PRMB), China. In this study, the effects of deposition processes in submarine canyons and the distribution of gas chimneys on gas hydrate accumulation were investigated using high-resolution two- dimensional (2D) and three-dimensional (3D) seismic data. Four intact submarine canyons were identified in the study area. Five deepwater depositional elements are closely related to submarine canyons: lateral accretion packages (LAPs), basal lags, slides, mass transport deposits (MTDs), and turbidity lobes. MTDs and lobes with multiple stages outside the distal canyon mouth reveal that the sedimentary evolution of the canyon was accompanied by frequent sediment gravity flows. Gas chimneys originating from Eocene strata are generally up to 3 km wide and distributed in a lumpy or banded pattern. The analysis of seismic attributes confirmed fluid activity in these gas chimneys. Gas hydrates are mainly distributed in ridges among different canyons. Based on the gas sources of gas hydrates and depositional evolution of submarine canyons, depositional processes of sediment gravity flows in submarine canyons and the distribution of gas chimneys significantly affect the accumulation of gas hydrates. Based on these findings, this study establishes a conceptional model for the accumulation of gas hydrate, which can provide guidance in the prediction for favorable gas hydrates zones in the area and nearby.

In this paper we present geomorphological hazard mapping of the San Vito Peninsula offshore (Sicilian margin of Tyrrhenian Sea), characterised by a very narrow continental shelf and a very dipping, tectonically active continental slope, no far from a seismogenic belt. The data set consists of morpho-bathymetric models achieved by means of multibeam survey and scattered high resolution seismic profiles. The morpho-bathymetric study highlighted some potential sites of geomorphological hazards, the most representative of which are: (i) two canyons developing from the upper slope off San Vito Cape down to the Erice basin; (ii) a dense network of small incisions across the upper slope; (iii) a very active landslide, developed onland and extending in the continental shelf close to the Scopello village. The geomorphological features that can generate submarine hazard are represented by: unstable sediment packages into canyon heads or along the upper slope; submarine canyons with regressive erosion; coastal landslides. [ABSTRACT FROM AUTHOR]

Identifying vulnerable marine habitats is fundamental to promote evidence-based management and a sustainable use of our oceans. Although progress in the mapping of marine benthic habitats has been made, complex ecosystems such as submarine canyons remain largely unexplored, hampering evidence-based management from taking place in these ubiquitous features worldwide. Data from a remotely operated vehicle (ROV) survey in the upper São Vicente Submarine Canyon in southwestern Portugal from 2011, was analysed to carry out a comprehensive assessment of megabenthic habitats, particularly those of conservation concern occuring between 90 and 560 m. Underwater videos from three exploratory dives, covering a total of 8.8 kilometers, were analyzed through multivariate and visual assessments, and the identified habitats were assessed against a set of criteria derived from current definitions of priority habitats. Thirteen potential priority habitats were characterized, out of which four distinct “coral gardens” and one “deep-sea sponge aggregation” are aligned with OSPAR definitions of priority habitats, and one corresponds to the habitat of a strictly protected sea urchin listed in the Habitats Directive. Three circalittoral sponge aggregations were also suggested based on the abundance of indicator species. The specific composition and diversity of priority habitats as described here, has not yet been described in any other canyon in the NE Atlantic region. Depth was revealed to be particularly important for the distribution of habitats, followed by relief and substratum. The approach presented in this study contributes to improve the understanding about submarine canyons, which is paramount to implement knowledge-based protection measures. Furthermore, it can be used to support future mapping efforts of deep-sea environments such as submarine canyons.

The submarine canyons are an important clue to study the evolution process of seafloor geomorphology and they generally indicate the significant linear grooves on the seafloor related to seafloor geodynamic erosion during the evolution of geomorphology. The submarine canyons or canyon groups are not only the channels for the sediment transport from shallow sediments with land-based sources to the deep sea in the sediment source-sink system, but also a key temporary sediment deposition area to study sediment transport patterns and the evolution of submarine geomorphology. In this paper, we processed and analyzed the multibeam bathymetry data acquired in the South China Sea continental margin by the research vessel “Dongfanghong 3” in 2020. Based on fine submarine geomorphological features identified from multibeam bathymetry data, we construct the formation pattern of the cyclic steps. The six cyclic steps (wavelengths of 1–6 km and wave heights of 19–81 m) are found in the lower section of the Penghu canyon and they appeared at the conjunction part of the Penghu and the Taiwan canyon. Based on location and the wavelength variations of the cyclic steps, we propose that the cyclic steps are formed by turbidity current flow along the Penghu and the Taiwan canyons. The axis of the cyclic step CS4–CS6 is shifted westward by about 5° compared to the axis of the cyclic step CS1–CS3. The inconsistency in the axis direction of the cyclic steps CS1–CS3 and CS4–CS6 suggests that is where the migration of the Penghu canyon occurred.

Synchronous interaction between bottom currents and turbidity currents has been reported often in channel–levée systems where the thickness of the turbidity currents exceeds that of the levées. Such interplay between along‐slope and down‐slope sedimentary processes is one of the mechanisms by which 'mixed turbidite–contourite systems' can originate. However, bottom currents flow over large areas of the seafloor, including continental slopes characterized by deeply incised submarine canyons rather than channel levées. In these cases, a direct interaction between along‐slope and down‐slope currents is, theoretically, unlikely to take place. In this study, oceanographic, swath bathymetry, multichannel seismic data and sediment cores are used to investigate a 25 km long, 10 km wide and up to 0.5 km thick deep‐sea late Quaternary deposit that sits adjacent to the north‐west flank of one of the major canyons in the North Atlantic, the São Vicente Canyon, in the Alentejo Basin (south‐west Iberian margin). The area receives the influence of a strong bottom current, the Mediterranean Outflow Water, which has swept the continental slope at different water depth ranges during glacial and interglacial periods. Architectural patterns and sediment characteristics suggest that this sedimentary body, named Marquês de Pombal Drift, is the result of the interaction between the Mediterranean Outflow Water (particularly during cold periods) and turbidity currents flowing along the São Vicente Canyon. Because the canyon is incised significantly deeper (ca 1.5 km) than the thickness of turbidity currents, an additional process, in comparison to earlier models, is needed to allow the interaction with the Mediterranean Outflow Water and transport sediment out of the canyon. In the São Vicente Canyon, and likely in other canyons worldwide, interaction of turbidity currents with contour currents requires intermediate nepheloid layers that export the finer‐grained fraction of turbidity currents out of the canyon at the boundary between major water masses. [ABSTRACT FROM AUTHOR]

To clarify the Tsushima Warm Current (TWC) system around a downstream region of Noto Peninsula (NP) where Toyama Trough (TT) causes a discontinuity of the along shelf-current, numerical experiments were performed using two-layered ocean model with simplified bottom topography. When a current trapped by the continental slope representing an offshore branch of TWC (OB) encounters the NP and TT, the OB was trapped by the coast facing the TT. A clockwise lee-eddy developed between a current axis and a coastline over the TT, which results in a current path transition offshore (CPT). The OB finally adjusted to a current path which crosses TT by shifting from the discontinuity point of the continental slope. If a coastal current representing a coastal branch of the TWC (CB) developed under this situation, the CB also formed lee-eddy within the identical region of the OB's eddy, resulting in CPT. Period for the CPT of the CB was shorter with increase of the OB volume transport. These results indicate that the OB acted to accelerate CPT of the CB within the TT. To support these results, observational data comprising mooring current, tidal, and CTD measurements obtained in and around the TT, along with the output from high-resolution data-assimilated ocean model DR_C were analyzed. Although the lee-eddy of the CBs was generated in every summer, the CPT events did not fully develop when the OB was absent over the continental slope because of the slow growth rate of the lee-eddy. [ABSTRACT FROM AUTHOR]